This invention relates to a flow rate measuring apparatus, and more particularly to a flow rate measuring apparatus for measuring a flow rate of fluid varying or fluctuating such as exhaust gas discharged from an internal combustion engine such as an automobile engine or the like.
There has been conventionally known an ultrasonic flowmeter or flow rate measuring apparatus adapted to measure a flow rate of fluid by means of an ultrasonic wave. Such an ultrasonic flowmeter is constructed so as to measure a flow rate of exhaust gas discharged from an automobile engine by transmission/reception of an ultrasonic wave and applied to measurement of performance of an automobile engine, environmental protection or the like. Such a conventional ultrasonic flowmeter is adapted to measure a flow rate at predetermined intervals within each of measuring sections.
Now, such a conventional ultrasonic flowmeter or flow rate measuring apparatus will be described with reference to FIGS. 6 and 7, wherein FIG. 6 is a block diagram of the flow rate measuring apparatus and FIG. 7 is a diagram showing waveforms of a flow rate during a periodic variation in flow rate in the flow rate measuring apparatus.
The conventional flowmeter, as shown in FIG. 6, includes a flow rate detection section 104 constituted by a thermal flow sensor or the like. The flow rate detection section 104 is mounted on a fluid pipe 110b and adapted to generate an output signal. The output signal of the flow rate detection section 104 is fed to a signal processing section 103, which digitizes the output signal. The signal processing section 103 has an averaging section 105 connected thereto, which is adapted to average the digitized output signal of the signal processing section 103, when any periodic variation occurs in flow of fluid in the fluid pipe 110b. The averaging section 105 has a flow rate operation section 106, which is adapted to calculate an average flow rate of the fluid using the averaged output signal. The flow rate measuring apparatus, as shown in FIG. 7, is adapted to carry out sampling of the flow rate in an intermittent manner, so that flow rates Q1, Q2 and Q3 measured at times t1, t2 and t3 may be averaged, leading to calculation of the flow rate.
However, the conventional ultrasonic flowmeter fails to accurately measure a flow rate of fluid pulsating at a relatively high frequency such as exhaust gas discharged from an internal combustion engine or the like, so that it is required to carry out measurement over a long period of time in order to calculate an average of the flow rate. Also, the conventional ultrasonic flowmeter causes a significant error to occur in measurement of an instantaneous flow rate, to thereby render measurement of a flow rate at a transient state substantially difficult or meaningless. Nevertheless, a tighter control of exhaust emission requires that an instantaneous flow rate and pulsating flow per se be measured or analyzed as well as an average of a flow rate.
Thus, it would be highly desirable to develop an ultrasonic flowmeter which is capable of measuring a typical flow rate of pulsating flow in a short period of time and reproducing pulsating flow.
The present invention has been made in view of the foregoing disadvantage of the prior art.
Accordingly, it is an object of the present invention to provide a flow rate measuring apparatus which is capable of accurately measuring a flow rate of fluid varying or fluctuating.
In accordance with the present invention, a flow rate measuring apparatus is provided. The flow rate measuring apparatus includes an ultrasonic transmission section for transmitting an ultrasonic wave to fluid flowing in a flow passage, and an ultrasonic receiving section for receiving an ultrasonic wave which has propagated through the fluid. The apparatus further includes a transmission timing control section constructed so as to permit the ultrasonic transmission section to transmit an ultrasonic wave in accordance with a plurality of transmission modes different in transmission timing from each other, and a flow rate operation section for calculating a flow rate of the fluid on the basis of an output signal of the ultrasonic receiving section.
In a preferred embodiment of the present invention, the flow rate measuring apparatus further includes a transmission mode selection section for carrying out change-over among the transmission modes based on a result of the calculation by the flow rate operation section.
In a preferred embodiment of the present invention, the transmission modes include a first transmission mode which permits an ultrasonic wave to be transmitted at a predetermined timing for every period of a flow waveform of the fluid and a second transmission mode which permits an ultrasonic wave to be transmitted at a timing shifted by a predetermined time for every period of the flow waveform of the fluid.
In a preferred embodiment of the present invention, the transmission modes further include a third transmission mode which permits an ultrasonic wave to be transmitted at predetermined intervals.
In a preferred embodiment of the present invention, the flow rate measuring apparatus further includes a transmission timing setting section for resetting a transmission timing of the first transmission mode in accordance with a result of the calculation by the flow rate operation section. The transmission mode selection section carries out change-over from the second transmission mode to the first transmission mode thus reset.
In a preferred embodiment of the present invention, the flow rate measuring apparatus further includes a flow rate variation judging section for judging whether or not a variation in flow rate is reduced based on a result of the calculation by the flow rate operation section. The transmission mode selection section carries out change-over from the first or second transmission mode to the third transmission mode when a variation in flow rate is reduced.